1. Field of the Invention
This invention relates to processes and apparatus for producing hydrocarbon products from oil shale in a rotary hearth. In preferred embodiments this invention pertains to systems for obtaining hydrocarbons from oil shale in a rabbled rotary hearth using heated spent oil shale as a heat exchange medium. In one embodiment the spent oil shale is heated with compatible combustible solids, such as coal char.
2. Prior Art
Oil shale contains hydrocarbons which can be recovered in the form of kerogen and volatile gases. These hydrocarbons are removed from the shale by subjecting the shale to heating and retorting. As the supply of naturally occurring accessible liquid and gaseous hydrocarbons diminishes, it is becomming necessary to rely either upon more and more foreign produced hydrocarbons, or to find sound economical methods of obtaining hydrocarbons from other resources. The most abundantly available hydrocarbon resource is oil shale.
Many systems, including some using a heat exchange medium, have been proposed for use in the recovery of hydrocarbons from oil shale. In one system oil shale in the form of fines is passed through a fluidized bed and heated to drive off hydrocarbons. However, such systems are inefficient, yielding only a relatively small percentage of hydrocarbons from the shale, and some of the hydrocarbons which are derived being subjected to cracking which produces less desirable hydrocarbons. In another system, solid oil shale is dropped into the top of a vertical retort through which hot gases are passed. This causes oil and volatile gases to be driven from the oil shale as it passes downward through the retort. While this technique can provide substantially complete removal of hydrocarbons from the shale, it again results in some cracking in the retort of the hydrocarbons removed from the shale.
In other prior art heat transfer systems, heat is transferred to particulate oil shale from a heated inert material, such as hot steel balls, sand or quartz. While this provides a fairly efficient method of heat transfer, it also requires separation of the heated inert material from the spent oil shale for further use of the inert material as a heat exchange medium.
In several systems a rotary kiln, has been utilized to process oil shale or coking coal with heat transfer from spent shale or coal. For example, in U.S. Pat. No. 2,723,226 a stream of fine sized coking coal or oil shale is projected into a mass of previously heated coke or char fines in a mixing screw and the mixture then projected into a revolving rotary retort kiln. This system requires both that the starting raw material be of substantially fine size, and further requires that careful and complex valving control and a separate mixing screw device be used in order to control feeding the fines into the rotating kiln. In U.S. Pat. No. 3,496,094 raw oil shale is mixed with hot solid spent oil shale in a rotating kiln in which the kiln includes baffles oriented to cause the solids to mix. The spent oil shale is preheated utilizing expensive volatile hydrocarbons.